347S31 Forgings: Niobium-Stabilised Stainless Rings, Flanges, Bars and Shafts
Jiangyin Jiangnan Metal Co., Ltd. forges 347S31 rings, seamless rolled rings, flanges, round bars, shafts, discs, sleeves, bushings and tube sheets to customer drawings at its open-die shop in Jiangyin, Jiangsu, China.
347S31 is the British Standard designation, BS 970-3:1991, for a niobium-stabilised 18Cr-10Ni austenitic stainless steel. It is equivalent to AISI 347, UNS S34700 and EN 1.4550 (X6CrNiNb18-10). Niobium fixes carbon as niobium carbide, so chromium carbides do not precipitate at the grain boundaries. The result is resistance to intergranular corrosion after welding and after sensitising exposure, together with creep strength and oxidation resistance at elevated temperature.
Forgings are supplied solution annealed at 1020 to 1100 C with rapid cooling. In softened bar below 160 mm the minimum values are 510 MPa tensile strength, 205 MPa yield strength and 30 % elongation.
What 347S31 is
347S31 sits in the 18Cr-10Ni austenitic family. It differs from plain 304 by a niobium addition of at least ten times the carbon content, up to 1.00 %. Niobium has a stronger affinity for carbon than chromium does, so carbon is locked into stable niobium carbides rather than forming chromium-rich carbides along the grain boundaries. That removes the sensitisation mechanism behind weld decay, which is why fabricators specify this grade for welded and thermally cycled assemblies instead of simply dropping to a low-carbon grade.
Against the titanium-stabilised alternative, 321 and 321H, the niobium carbides hold to higher temperature, so 347S31 keeps useful strength and scaling resistance in hot service. The steel is non-magnetic when annealed, weldable without mandatory post-weld heat treatment, and tough at low temperature. Published data for the equivalent 1.4550 grade show impact energy above 70 J/cm2 at minus 253 C.
Equivalent grades
| Region or system | Designation |
|---|---|
| United Kingdom, BS | 347S31, 347S17 |
| USA, ASTM / AISI | 347, TP347 |
| USA, UNS | S34700 |
| Europe, EN | X6CrNiNb18-10, material no. 1.4550 |
| Germany, DIN | X6CrNiNb18-10 |
| France, NF | Z6CNNb 18-10 |
| Japan, JIS | SUS 347 |
| Korea, KS | STS 347 |
| China, GB | 06Cr18Ni11Nb, S34778 |
| Russia, GOST | 08Kh18N12B |
| Sweden, SS | 2338 |
| ISO | X6CrNiNb 18-10 |
Chemical composition
| Element | C | Si | Mn | P | S | Cr | Ni | Nb | Cu | Mo | Ti |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Min | – | – | – | – | – | 17.0 | 9.0 | 10 × C | – | – | – |
| Max | 0.08 | 1.00 | 2.00 | 0.045 | 0.030 | 19.0 | 12.0 | 1.00 | 0.70 | 1.00 | 0.10 |
Where the order calls for AISI 347 or UNS S34700 instead, the ranges widen to 17.0 to 20.0 % Cr, 9.0 to 13.0 % Ni and Nb max 1.10 %. We melt to either window. State the controlling specification on the enquiry.
Mechanical properties
| Condition | Size | Tensile Rm, MPa | Yield Re, MPa | Elongation, % |
|---|---|---|---|---|
| Cold drawn | d up to 19 mm | 865 | 695 | 12 |
| Cold drawn | 19 to 25 mm | 790 | 555 | 15 |
| Cold drawn | 25 to 32 mm | 725 | 450 | 20 |
| Cold drawn | 32 to 38 mm | 695 | 340 | 28 |
| Cold drawn | 38 to 45 mm | 650 | 310 | 28 |
| Annealed, final cold drawn | d up to 19 mm | 600 | 375 | 15 |
| Annealed, final cold drawn | d over 19 mm | 550 | 325 | 20 |
| Softened, solution annealed | d below 160 mm | 510 | 205 | 30 |
Physical data and hot strength
| Property | 20 C | 100 C | 200 C | 300 C | 400 C | 500 C |
|---|---|---|---|---|---|---|
| Yield strength Re, MPa | 205 | 175 | 155 | 136 | 125 | 119 |
| Modulus of elasticity, GPa | 200 | 194 | 186 | 179 | 172 | 165 |
| Linear expansion, 10-6 per K | – | 16.0 | 16.5 | 17.0 | 17.5 | 18.0 |
Density is about 7.90 kg/dm3. Thermal conductivity is about 15 W/m.K and specific heat about 500 J/kg.K at 20 C. Annealed hardness runs up to about 230 HB.
Heat treatment
Solution anneal at 1020 to 1100 C with a short soak, then cool rapidly in water or forced air. This dissolves residual carbides and restores full corrosion resistance. Stress relieve at 850 to 950 C for 10 to 15 minutes and cool in air. The grade takes no hardening heat treatment. Strength is raised only by cold work, and the alloy work-hardens quickly. Because niobium has already fixed the carbon, welded 347S31 does not normally call for post-weld solution annealing.
Melting, forging and inspection
We melt 347S31 by electric arc furnace with vacuum oxygen decarburisation, EAF plus VOD. Electroslag remelting is available where the order sets tighter cleanliness, segregation control or through-thickness soundness. Forging runs on open-die hammers and presses. Ring blanks are produced as seamless rolled rings so that grain flow follows the circumference, which is what puts forged rings ahead of cut-and-welded plate rings in fatigue and pressure service.
Ultrasonic examination is offered to EN 10228-3, SEP 1921 or ASTM A388, with the acceptance class agreed at order stage. Certificates are issued to EN 10204 3.1, or EN 10204 3.2 with third party witness and endorsement. Further testing on request covers chemical analysis, tensile and impact tests, hardness survey, intergranular corrosion test, PMI, dye penetrant and a dimensional report.
Product forms
| Form | Typical use | Dimensions |
|---|---|---|
| Seamless rolled rings, forged rings | Flange rings, retaining rings, bearing races, valve seat rings | State OD x ID x height |
| Forged flanges | Pipeline, heat exchanger and pressure vessel connections | To drawing, or ASME B16.5 / B16.47 |
| Forged round bars and shafts | Pump and compressor shafts, spindles, eccentric shafts, valve stems | State diameter x length |
| Forged discs and disks | Blind covers, gear blanks, turbine and blower parts | State diameter x thickness |
| Forged sleeves and bushings | Wear sleeves, liners, housings | State OD x ID x length |
| Forged tubes and hollow bars | Cylinders, nozzle bodies | State OD x ID x length |
| Forged tube sheets | Shell and tube heat exchangers, condensers | State diameter x thickness |
| Forged blocks and valve bodies | Valve blocks, manifolds, die blocks | To drawing |
Supply condition options are as-forged, rough machined with ultrasonic test allowance, or finish machined to drawing tolerance.
Service applications
Chemical and petrochemical plant. Nitric acid plant, salts and fertiliser equipment, autoclaves, absorption towers, reactors, coolers and pump parts. The grade holds up in nitric acid to roughly 55 % concentration and about 80 C.
Oil, gas and refining. Forged flanges, tube sheets, valve bodies, seat rings, stems and separator internals, including refinery units in polythionic acid service where stabilised grades are specified because sensitisation has to be ruled out.
Power and high temperature plant. Expansion joints, superheater and manifold parts, turbine and gas compressor components, gearbox and power transmission forgings.
Heat exchange and cryogenic duty. Exchanger and condenser tube sheets, transport tanks, pipeline forgings, and low temperature service where toughness decides the grade.
Food, pharmaceutical, pulp and paper. Processing and storage equipment, dairies, sugar plants and breweries, pulp digesters, liquor tanks and paper machine parts.
Questions we are asked about 347S31
What is 347S31 equivalent to?
AISI and ASTM 347, UNS S34700, EN 1.4550 / X6CrNiNb18-10, JIS SUS 347, KS STS 347, GB 06Cr18Ni11Nb (S34778), NF Z6CNNb 18-10, GOST 08Kh18N12B and SS 2338. BS 347S17 is the related British designation.
Is 347S31 magnetic?
In the solution annealed condition it is essentially non-magnetic. Heavy cold work or machining can raise a slight magnetic response at the worked surface.
347S31 or 321: which applies?
Both are stabilised 18-8 grades. 321 uses titanium, 347S31 uses niobium. Niobium carbides stay stable to higher temperature, so 347S31 is the usual choice for hotter service and for weld consumable compatibility. Titanium is also lost more readily across the arc, which is why 347 type consumables are common even on 321 base metal.
Can you supply 347H instead?
Yes. 347H is the higher carbon variant, about 0.04 to 0.10 % C, used where creep strength above roughly 540 C governs the design. Send the design temperature and code case and we will confirm the grade.
What is the minimum order quantity?
We forge to order from single piece prototypes upward. Cost per piece drops once several parts share one melt and one heat treatment batch, so send the full drawing package if repeat quantities are planned.
What do you need in order to quote?
Grade and controlling specification, drawing or finished dimensions, quantity, supply condition, non-destructive testing and acceptance class, certificate type, and destination port.
Request a 347S31 forging quotation
Send a drawing or the finished dimensions and we will return price, forged weight and lead time.
Jiangyin Jiangnan Metal Co., Ltd., Open-Die Forging Factory
No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China
Telephone +86-189-2135-9659
Email sales@steelforgepieces.com
Related stainless forging grades
Reference standards
Composition and mechanical limits on this page follow BS 970-3:1991, specification for wrought steel for mechanical and allied engineering purposes, part 3, bright bars for general engineering purposes. Physical data, hot strength, low temperature toughness and the cross-reference designations follow the equivalent EN 10088-1 grade X6CrNiNb18-10, material number 1.4550. Figures are for guidance. The governing order specification and the mill certificate take precedence. Page last reviewed 28 September 2026.